1. Field of the Invention
The present invention relates to an optical duo-binary transmission apparatus using an optical duo-binary transmission method . More particularly, the present invention relates to a duo-binary encoder performing a parallel processing and an optical duo-binary transmission apparatus using the same.
2. Description of the Related Art
In general, a Dense Wavelength Division Multiplexing (hereinafter, referred to as a DWDM) optical transmission system has an excellent communication efficiency, since it can transmit an optical signal having multiple channels with different wavelengths through a single optical fiber. Also, the DWDM system is capable of transmitting an optical signal regardless of transmission speed. Accordingly, the DWDM systems are now widely used in ultra-high speed internet networks, there are trends showing more and more data traffic being sent over such networks Recently, there are known systems in use that are capable of transmitting more than a hundred channels through a single optical fiber using the DWDM technology. Furthermore, various research is being actively conducted to develop a system that can transmit more than two hundred channels of 40 Gbps through a single optical fiber simultaneously, thus having a transmission speed of more than 10 Tbps.
However, one restriction to the enlargement of transmission capacity is due to severe interference and distortion between the channels. Such interference can be present when the channel distance is less than 50 GHz when a light intensity is modulated using the conventional non-return-to-zero (NRZ) method, which is due to a rapid increase of data traffic and a request for high-speed transmission of data of more than 40 Gbps. Transmission distance is also restricted in high-speed transmission of more than 10 Gbps since a direct current (DC) frequency component of a conventional binary NRZ transmission signal and a high frequency component spread during modulation cause non-linearity and dispersion when the binary NRZ transmission signal propagates in an optical fiber medium.
Recently, optical duo-binary technology has been gaining prominence as an optical transmission technology capable of overcoming the restrictions associated with transmission distance due to chromatic dispersion. One primary advantage of the duo-binary transmission versus other forms of DWDM transmission is that the transmission spectrum is reduced when compared with general binary transmissions.
In addition, in a dispersion restriction system, a transmission distance is inversely proportional to the square of the transmission spectrum bandwidth. For example, when the transmission spectrum is reduced by ½, the transmission distance increases four times. Furthermore, since a carrier frequency is suppressed in a duo-binary transmission spectrum, it is possible to relax the restriction of an optical power output caused by Brillouin scattering excited in the optical fiber.
FIG. 1 is a block diagram showing one construction of a conventional optical duo-binary transmission apparatus. Hereinafter, the conventional optical duo-binary transmission apparatus will be described with reference to FIG. 1.
In FIG. 1, the conventional optical duo-binary transmission apparatus includes a multiplexer 10, a precoder 20, a low pass filter 30, a modulator driving amplifier 40, a laser source 50 for outputting a carrier, and a Mach-Zehnder interferometer type optical intensity modulator 60. The multiplexer 10 multiplexes data input signals of N number of channels 1−N so as to output the multiplexed signal, and the precoder 20 encodes the multiplexed signal. The low pass filter 30 converts a two-level NRZ electrical signal output from the precoder 20 into a 3-level electrical signal, and reduces the bandwidth of the signal. The modulator-driving amplifier 40 amplifies the 3-level electrical signal so as to output an optical modulator-driving signal.
In typical operation, the input signals of N number of channels are multiplexed by the multiplexer 10, and the multiplexed signal is then encoded by the precoder 20. The 2-level binary signal output from the precoder 20 is input to the low pass filter 30, and the low pass filter 30 has a bandwidth corresponding to about ¼ of a clock frequency of the 2-level binary signal. This excessive limitation to the bandwidth causes interference between codes, which thus changes the 2-level binary signal to a 3-level duo-binary signal.
Further, the 3-level duo-binary signal is amplified by the modulator-driving amplifier 40 so as to be used as a driving signal of the Mach-Zehnder interferometer type optical intensity modulator 60. The carrier output from the laser source 50 is subject to phase and optical intensity modulation according to the driving signal of the Mach-Zehnder interferometer type optical intensity modulator 60 and is then output as a 2-level optical duo-binary signal.
FIG. 2 is a view showing a pattern and a phase shift of an output optical signal when a signal having a data sequence of 0011 0101 1110 1010 (35EA) is transmitted by means of the conventional optical duo-binary transmission apparatus shown in FIG. 1. In FIG. 2, whenever the data input signal becomes ‘0’, the phase of the data input signal is shifted by π.
However, according to prior art devices as shown in FIG. 1, in generating the 3-level data signal by the electric low pass filter, a pseudo random bit sequence (hereinafter, referred to as a PRBS) has large influence. As the length of PRBS increases, a deterioration of transmission characteristics also increases, thereby causing much difficulty in realizing the system.
Furthermore, according to prior art devices as shown in FIG. 1, the input data are multiplexed, and the multiplexed data are then encoded by a precoder. Thus, the speed of the precoder must simultaneously increase as the transmission speed of the data increases. However, in a case of the conventional precoder, the structure includes san exclusive OR (XOR) gate and a time delay unit for delaying an output signal of the XOR gate by 1 data bit and feed backing the delayed signal. Therefore, when using high-speed data signals, it is difficult to operate a high-speed precoder due to time delay and limitation in speed of the XOR gate.
FIG. 3 is a block diagram showing another construction of a conventional optical duo-binary transmission apparatus. FIG. 4 shows output signals at points {circle around (1)}, {circle around (2)}, {circle around (3)}, {circle around (4)}, and {circle around (5)} in FIG. 3.
In FIG. 3, the conventional optical duo-binary transmission apparatus includes a multiplexer 10, an encoder 70, a coupler or an adder 80, a modulator-driving amplifier 40, a laser source 50 for outputting a carrier, and a Mach-Zehnder interferometer type optical intensity modulator 60. The multiplexer 10 multiplexes data input signals of N number of channels so as to output the multiplexed signal, and the encoder 70 encodes the multiplexed signal so that the multiplexed signal includes phase information. The coupler 80 converts the encoded signal into a 3-level electrical signal, and the modulator-driving amplifier 40 amplifies the 3-level electrical signal so as to output an optical modulator-driving signal.
According to the conventional optical duo-binary transmission apparatus shown in FIG. 3, neither a low pass filter nor a precoder are used. Instead, in order to enable the apparatus to have a phase shift that is a main characteristic of an optical duo-binary signal, the encoder 70 outputs data {circle around (2)} having non-shifted phases and data {circle around (3)} requiring a phase shift, from multiplexed data signals {circle around (1)} outputted from the multiplexer 10.
The output signals {circle around (2)} and {circle around (3)} of the encoder 70 are converted into a 3-level signal {circle around (4)} by the coupler 80, and the converted signal is passed through the optical intensity modulator 60 via the driving amplifier 40 and is then output as an optical duo-binary signal {circle around (5)} with a phase shift.
Similar to the apparatus in FIG. 1, since the optical duo-binary transmission apparatus in FIG. 3 multiplexes the input signals of N number of channels and then encodes the multiplexed signal, the apparatus requires a high-speed encoder. However, it is difficult to realize such a high-speed encoder, due to limitations in speed of electrical components constituting the encoder.